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TPD3S014-Q1: output capacitor

Sherry Liang
Genius 10175 points
Part Number: TPD3S014-Q1

Dear team,

I have a question for the output capacitor value. In the below section, our datasheet explains why we need 150uF capacitor, but I still can't understand it. 

Firstly, based on the description, I think 'a short' means the load shorts to ground. Once short occurs, the current will abrupt increase, and our device will enter protection mode, and then the current will be decreased to 0.85A. So my first concern is that the output current will be increased firstly, not decreased.

Secondly, assuming the current is decreased, then the inductance for the USB cable will generate back electromotive force, so the output voltage will be decreased, maybe negative, right?

Thirdly, the datasheet said that 120uF is a must for USB standard applications. Could you please tell me the reason? If the customer can accept the undershoot when short occurs, then 120uF is no need and 10uF is enough, right?

Thanks & Best Regards,

Sherry

  • 0
    TI__Expert 3890 points

    Hi Sherry,

    The output cap-150uF is from old USB spec. USB spec calls for some amount of output capacitance on ports - generally accepted to be 100uF min (120uF/150uF are popular values).  But there is not requirement for new USB spec. Thus, it can use 10uF+0.1uF if customer does not very care load transient performance.

    Regards,

    Ansel

  • 0 in reply to Ansel Song
    TI__Genius 10175 points

    Hi Ansel,

    Thanks for your reply!

    Load transient can result in output drop, and I can understand this point. I think our device can't response the load transient quickly, so there is a output voltage drop instead of the inductance of cable impact, right?

    Thanks & Best Regards,

    Sherry

  • 0 in reply to Sherry Liang
    TI__Expert 3890 points

    Hi Sherry,

    It is the both. Because it helps to decouple the inductance of the long wires with bigger caps.

    Regards,

    Ansel